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Deploying Flows

Reminder to start a Prefect API

Some features in this tutorial, such as scheduling, require a Prefect server to be running; if using a self-hosted setup, simply run prefect server start to run both the webserver and UI; if using Prefect Cloud, make sure you have successfully configured your local profile.

Why deployments?

One of the most common reasons to use a tool like Prefect is scheduling or event-based triggering. Up to this point, we’ve demonstrated running Prefect flows as scripts, but this means you have been the one triggering and managing flow runs. You can certainly continue to trigger your workflows in this way and use Prefect as a monitoring layer for other schedulers or systems, but you will miss out on many of the other benefits and features that Prefect offers.

Deploying a flow exposes an API and UI so that you can:

  • trigger new runs, cancel active runs, pause scheduled runs, customize parameters, and more
  • remotely configure schedules and automation rules for your deployments
  • dynamically provision infrastructure using workers

What is a deployment?

Deploying a flow is the act of specifying when, where, and how it will run. This information is encapsulated and sent to Prefect as a deployment that contains the crucial metadata needed for remote orchestration. Deployments elevate workflows from functions that you call manually to API-managed entities.

Attributes of a deployment include (but are not limited to):

  • Flow entrypoint: path to your flow function
  • Schedule or Trigger: optional schedule or triggering rule for this deployment
  • Tags: optional text labels for organizing your deployments

Create a deployment

Using our get_repo_info flow from the previous sections, we can easily create a deployment for it by introducing one simple line of code:

repo_info.py
import httpx
from prefect import flow


@flow(log_prints=True)
def get_repo_info(repo_name: str = "PrefectHQ/prefect"):
    url = f"https://api.github.com/repos/{repo_name}"
    response = httpx.get(url)
    response.raise_for_status()
    repo = response.json()
    print(f"{repo_name} repository statistics 🤓:")
    print(f"Stars 🌠 : {repo['stargazers_count']}")
    print(f"Forks 🍴 : {repo['forks_count']}")


if __name__ == "__main__":
    get_repo_info.serve(name="my-first-deployment")

Running this script will do two things:

  • create a deployment called "my-first-deployment" for your flow in the Prefect API
  • stay running to listen for flow runs for this deployment; when a run is found, it will be asynchronously executed within a subprocess

Deployments must be defined in static files

Flows can be defined and run interactively, that is, within REPLs or Notebooks. Deployments, on the other hand, require that your flow definition be in a known file (which can be located on a remote filesystem in certain setups).

Because this deployment has no schedule or triggering automation, you will need to use the UI or API to create runs for it. Let's use the CLI (in a separate terminal window) to see what happens:

prefect deployment run 'get_repo_info/my-first-deployment'

If you are watching either your terminal or your UI, you should see the newly created run execute successfully!
Let's take this example further by adding a schedule and additional metadata.

Additional options

The serve method on flows exposes many options for your deployment. Let's use a few of these options now:

  • cron: a keyword that allows us to set a cron string schedule for the deployment; see schedules for more advanced scheduling options
  • tags: a keyword that allows us to tag this deployment and its runs for bookkeeping and filtering purposes
  • description: a keyword that allows us to document what this deployment does; by default the description is set from the docstring of the flow function, but we did not document our flow
  • version: a keyword that allows us to track changes to our deployment; by default a hash of the file containing the flow is used; popular options include semver tags or git commit hashes

Setting these fields is simple: 

if __name__ == "__main__":
    get_repo_info.serve(
        name="my-first-deployment",
        cron="* * * * *",
        tags=["testing", "tutorial"],
        description="Given a GitHub repository, logs repository statistics for that repo.",
        version="tutorial/deployments",
    )

When you rerun this script, you will find an updated deployment in the UI that is actively scheduling work! Stop the script using CTRL+C and your schedule will be automatically paused.

.serve is a long-running process

In order for remotely triggered or scheduled runs to be executed, your script with flow.serve must be actively running.

Running multiple deployments at once

This method is useful for creating deployments for single flows, but what if we have two or more flows? This only requires a few additional method calls and imports to get up and running:

multi_flow_deployment.py
import time
from prefect import flow, serve


@flow
def slow_flow(sleep: int = 60):
    "Sleepy flow - sleeps the provided amount of time (in seconds)."
    time.sleep(sleep)


@flow
def fast_flow():
    "Fastest flow this side of the Mississippi."
    return


if __name__ == "__main__":
    slow_deploy = slow_flow.to_deployment(name="sleeper", interval=45)
    fast_deploy = fast_flow.to_deployment(name="fast")
    serve(slow_deploy, fast_deploy)

A few observations are in order:

  • the flow.to_deployment interface exposes the exact same options as flow.serve; this method produces a deployment object
  • the deployments are only registered with the API once serve(...) is called
  • when serving multiple deployments, the only requirement is that they share a Python environment; they can be executed and scheduled independently of each other

You should spend some time experimenting with this setup; a few next steps for exploration include:

  • pausing and unpausing the schedule for the "sleeper" deployment
  • use the UI to submit ad-hoc runs for the "sleeper" deployment with different values for sleep
  • use the UI to cancel an active run for the "sleeper" deployment (good luck cancelling the "fast" one 😉)

Next Steps

Congratulations! You now have your first working deployment. You can pause here and begin exploring Prefect's features, or take your understanding of Prefect deployments even further: